Photosystem II (PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water:plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-A resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per monomer. The presence of a third plastoquinone Q(C) and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.

Background: Mn 4 CaO 5 cluster catalyzes water oxidation in photosystem II. Results: Mn-Mn/Ca/ligand distances and changes in the structure of the Mn 4 CaO 5 cluster are determined for the intermediate states in the reaction using x-ray spectroscopy. Conclusion: Position of one bridging oxygen and related geometric changes may be critical during catalysis. Significance: Knowledge about structural changes during catalysis is crucial for understanding the O-O bond formation mechanism in PSII.

Using the 2.9 A resolution structure of the membrane-intrinsic protein-cofactor complex photosystem II (PSII) from the cyanobacterium Thermosynechococcus elongatus, we calculated and characterized nine possible substrate/product channels leading to/away from the Mn(4)Ca cluster, where water is oxidized to dioxygen, protons, and electrons. Five narrow channels could function in proton transport, assuming that no large structural changes are associated with water oxidation. Four wider channels could serve to supply water to or remove oxygen from the Mn(4)Ca cluster. One of them might be regulated by conformational changes of Lys134 in subunit PsbU. Data analyses of Kr derivatized crystals and complexes with dimethyl sulfoxide (DMSO) confirm the accessibility of the proposed dioxygen channels to other molecules. Results from Xe derivatization suggest that the lipid clusters within PSII could serve as a drain for oxygen because of their predominant hydrophobic character and mediate dioxygen release from the lumen.

The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.

The photosynthetic oxygen-evolving photosystem II (PSII) is the only known biochemical system that is able to oxidize water molecules and thereby generates almost all oxygen in the Earth's atmosphere. The elucidation of the structural and mechanistic aspects of PSII keeps scientists all over the world engaged since several decades. In this Minireview, we outline the progress in understanding PSII based on the most recent crystal structure at 2.9 A resolution. A likely position of the chloride ion, which is known to be required for the fast turnover of water oxidation, could be determined in native PSII and is compared with work on bromide and iodide substituted PSII. Moreover, eleven new integral lipids could be assigned, emphasizing the importance of lipids for the perfect function of PSII. A third plastoquinone molecule (Q(C)) and a second quinone transfer channel are revealed, making it possible to consider different mechanisms for the exchange of plastoquinone/plastoquinol molecules. In addition, possible transport channels for water, dioxygen and protons are identified.